1. Field of the Invention
The present invention relates to a thrust bearing for an automotive turbocharger.
2. Discussion of the Background
In recent years, automotive turbochargers have been increasingly used, and they are installed even in some subcompacts. There is now an increased demand for smaller and lighter turbochargers. Also, in order to operate the turbocharger efficiently from a lower rotational frequency of an engine, the rotational frequency of the turbocharger has tended to be increased. Further, there is a demand for a turbocharger that rotates at a quite high speed of 250,000 rmp. A thrust bearing having special functions is therefore needed to attain rotation of such a high speed.
Specifically, the thrust bearing must be strong enough to withstand a moving load which is applied axially of the rotor shaft of a turbocharger. Further, the bearing must be shaped so as not to adversely affect the performance of the oil seal on the side of a compressor. Conventional thrust bearings are shown in FIGS. 1 and 2 in front elevation. The bearing shown in FIG. 1 takes a circular form and has a plurality of surfaces 2 to which thrust load is applied. The surfaces 2 are formed around a hole 1 extending along the axis. In the bearing shown in FIG. 2, the lower portion is cut out so that an access can be had to a hole 1 extending along the axis to facilitate its installation on a rotor shaft. Either bearing is also formed with an oil passage 3.
The thrust bearing shown in FIG. 1 has a large surface area, because it is circular in shape. Therefore, oil tends to adhere to the surface, and it cannot be readily removed. The bearing shown in FIG. 2 has a smaller surface area because of the cutout in its lower portion, to get rid of the drawback with the bearing shown in FIG. 1. However, air within the lower cutout is introduced into the bearing during rotation of it. The air then accumulates in the form of bubbles within a chamber which should be filled with oil. Thus, the volume occupied by oil is reduced, creating deteriorations of the performance of the oil seal, including lack of lubricating oil on the thrust bearing.